There was no C++ to JS compiler used, it was Java compiled to Closure Compiler (J2CL) and Java compiles to Objective-C (j2objc) However only business logic is

There was no C++ to JS compiler used, it was Java compiled to Closure Compiler (J2CL) and Java compiles to Objective-C (j2objc) However only business logic is transpiled, the UI layer on every platform is written by hand in the native impedance matched language of the platform.